A three-way catalytic converter for purifying an exhaust is disposed in the exhaust system of an engine. For catalytic converter to maintain good converting efficiency, it is usual to carry out feedback control by having an intake air-fuel mixture to the engine maintain a theoretical air-fuel ratio.
The air-fuel ratio feedback control employs an oxygen sensor (an air-fuel ratio sensor) for detecting an air-fuel ratio according to the concentration of oxygen contained in the exhaust. To ensure good response from the oxygen sensor, the oxygen sensor is disposed at, for example, a collecting portion of an exhaust manifold in the vicinity of a combustion chamber. The oxygen sensor detects the concentration of oxygen contained in the exhaust, and according to the detected concentration, it is determined whether an actual air-fuel ratio is rich or lean with respect to a theoretical air-fuel ratio (a target air-fuel ratio). According to the rich or lean determination, the feedback control adjusts the supply of fuel to the engine.
Since the oxygen sensor is disposed close to the combustion chamber in the exhaust system, the oxygen sensor is exposed to a high-temperature exhaust, which may thermally deteriorate the characteristics of the sensor. When the oxygen sensor is located at the collecting portion of the exhaust manifold, where the exhaust from respective cylinders are not yet sufficiently mixed together, the oxygen sensor hardly detects a mean air-fuel ratio of all cylinders. This may cause a fluctuation in the air-fuel ratio detecting accuracy. Although detective response is secured by placing the oxygen sensor in the vicinity of the combustion chamber, the air-fuel ratio feedback control employing the oxygen sensor alone cannot stabilize an air-fuel ratio control accuracy.
To solve this problem, it has been proposed to arrange another oxygen sensor on the downstream side of the catalytic converter in addition to the one disposed on the upstream side thereof, and carry out the air-fuel ratio feedback control according to values detected by the two oxygen sensors (Japanese Unexamined Patent Publication No. 58-48756).
Although the downstream oxygen sensor has poor response due to an O.sub.2 storage effect of the three-way catalytic converter (causing an output delay in the sensor because excessive oxygen remains when an actual air-fuel ratio is lean with respect to a theoretical air-fuel ratio and residual oxygen remains when the actual air-fuel ratio is rich), it can stably detect an air-fuel ratio at which the CO, HC and NOx converting efficiency of the three-way catalytic converter is best. The downstream oxygen sensor, therefore, can achieve accurate and stabilized detection by compensating for the deterioration of the upstream oxygen sensor.
Values detected by the two oxygen sensors may be independently used to carry out air-fuel ratio feedback control. Alternatively, a control quantity for air-fuel ratio feedback control carried out according to a value detected by the upstream oxygen sensor may be corrected such that an air-fuel ratio detected by the downstream oxygen sensor approaches a target air-fuel ratio. Namely, the upstream oxygen sensor ensures the response of air-fuel ratio control, while the downstream oxygen sensor secures control accuracy of the air-fuel ratio control, thereby precisely carrying out the air-fuel ratio feedback control.
According to the conventional air-fuel ratio control system employing two oxygen sensors, a fuel supply quantity to the engine is always directly updated according to the output of the downstream oxygen sensor. When the output characteristics of the upstream oxygen sensor change, the conventional system provides no correction target for adjusting the control to attain the target air-fuel ratio. This may cause a control overshoot, which will be explained below.
An output of the downstream oxygen sensor involves a large response delay compared with that of the upstream oxygen sensor. When the downstream oxygen sensor detects that a present air-fuel ratio is lean (rich) relative to a target air-fuel ratio, the conventional control directly corrects a fuel supply quantity to the engine, to solve the lean (rich) state. Even if an air-fuel ratio in the combustion chamber has already been inverted to a rich (lean) state from a lean (rich) state, the control for bringing an actual air-fuel ratio to the rich (lean) state is continued until the downstream oxygen sensor detects an inversion of the air-fuel ratio.
Just before an air-fuel ratio detected by the downstream oxygen sensor is inverted from rich to lean or from lean to rich, the overshoot phenomenon may occur to widely fluctuate the air-fuel ratios even if a mean air-fuel ratio is equal to the target air-fuel ratio. This overshoot may cause spikes of CO, HC, and NOx.
To solve these problems, an object of the invention is to prevent an overshoot of air-fuel feedback control caused by a detection response delay of an air-fuel ratio sensor disposed on the downstream side of a catalytic converter.
More precisely, when the output characteristics of an air-fuel ratio sensor disposed on the upstream side of the catalytic converter are deteriorated by heat, etc., a correction target value used for correcting the air-fuel ratio feedback control to attain a target air-fuel ratio is set according to a result of detection by the air-fuel ratio sensor disposed on the downstream side of the catalytic converter. The correction target value is compared with an actual value when correcting the control so that the control will no be excessively corrected beyond the correction target value, and the air-fuel ratios will not flutuate widely.
Another object of the invention is to prevent the correction target value from excessively responding to an air-fuel ratio detected by the downstream air-fuel ratio sensor and destabilizing.
Still another object of the invention is to prevent an actual value corresponding to the correction target value from being influenced by a temporary fluctuation in the air-fuel ratio feedback control, avoid a misjudgment of the air-fuel ratio feedback control, and preclude an excessive control correction.